1. Field of the Invention
The present invention related to a unit injector.
2. Description of the Related Art
The present applicant has proposed a unit injector comprising a plunger driven by an engine, a high pressure fuel chamber filled with fuel which is pressurized by the plunger, a fuel supply port open to the high pressure fuel chamber when the plunger is at the compressing operation start position, and closed by the plunger when the fuel compressing action by the plunger is started, a needle moved in accordance with the fuel pressure in the high pressure fuel chamber to open a valve opening when the fuel pressure exceeds a predetermined pressure, a variable volume chamber filled with fuel and having a volume which is changed by an actuator, a spill valve controlling the spillage of fuel in the high pressure fuel chamber in response to the fuel pressure in the variable volume chamber, a fuel charging port formed on the inner circumferential wall of a plunger bore and in communication with the variable volume chamber, and a fuel charging bore formed in the plunger. One end of the fuel charging bore is open at the outer circumferential wall of the plunger, and the other end of the fuel charging bore is open to the high pressure fuel chamber. When the plunger is at the fuel compressing operation start position, the fuel charging port is connected to the fuel charging bore, and while the fuel compressing action by the plunger is carried out the fuel charging port is closed by the plunger (Japanese Utility Model Application No. 63-1707).
In this unit injector, since the fuel charging port is connected to the fuel charging bore when the plunger is at the fuel compressing operation start position, the fuel in the high pressure fuel chamber is fed into the variable volume chamber via the fuel charging bore and the fuel charging port, whereby the variable volume chamber is charged with fuel. Then, while the fuel compressing action by the plunger is carried out, the fuel charging port is closed by the plunger.
However, when the fuel in the high pressure fuel chamber is pressurized and thus reaches a high pressure, a part of the fuel under high pressure leaks through the clearance between the plunger and the plunger bore. At this time, even if the fuel charging port is closed by the plunger, the thus leaked fuel under high pressure flows into the fuel charging port, and as a result, since the pressure of fuel is the variable volume chamber becomes high, the spill valve is actuated although the actuator is not operated, this result in a problem in that it becomes impossible to properly control the fuel injection by the actuator.